Monday, June 26, 2006

The SCR Controller, SCR Power Block, HV Transformer with current limiting resistor, HV rectifier diodes, charging and dump resistors, LED status lights, Trigger Inhibit boxes, and all control and coaxial cables that went to the main control rack were removed. These functions were replaced by a new dual charging board and two Gamma Power supplies as shown the right picture.

Each side of the dual charging board (see schematic on bottom of previous page) utilizes ten 10K, 100W Ohmite wirewound resistors that serve to isolate the Gamma power supplies and additionally provide energy dumps for both Thyratron Banks via the common Ross relay. Each charging side of the board is the same; only the North side will be described herein but the South side functions in the same manner. Resistors R3 through R10 are used to dump a maximum 675 joules of stored energy in the five 0.30 uF capacitors per bank when charged to 30 kV. Resistors R1 and R2 can be used to set the constant current charging current limit of the Gamma power supply when the Ross relay is left de-energized. Resistors R21 through R25 and monitor resistor R31 provide a 10,000:1 voltage divider to be terminated into 1 Megohms or greater for power supply calibration purposes. This divider is not resistor-capacitor compensated and is not intended to observe fast discharge pulses. The 50-Megohm-divider string also provides a slight bleed for the power supply to help hold the charge voltage accurately. The North and South Banks were made completely independent by removing the 3K, 250W Ohmite wirewound cross-connecting resistor and associated hardware in the top oil tank. The output of each new charging resistor string was connected to the North and South capacitor banks via a Dielectric Sciences coaxial, silicone based, semicon graded, high-voltage cable rated at 60 kV DC.

Both the North and South Banks were operated at a constant charging current of 12 ma and a charge voltage of 28 kV; this operation was successfully achieved only after much troubleshooting discussed later in this report. The constant current charging time can be estimated to be T = CV/I = (1.5uF)(28kV/(12ma) = 3.5 secs. In actuality, the charging time is a little greater due to the slight voltage drop across the charging resistor string as the power supply switches from constant current to constant voltage operation as the final charge voltage is reached. This power supply portion of Prometheus is only compatible with 0.2 Hz operations unless reduced power operations are invoked. The scope trace on the top of the next page shows the North and South Bank secondary voltages as well as the Thyratron primary currents.

Referring to the scope traces on the top of the next page, at first glance it appears that the transfer time of the North Bank (Red) appears to be faster than that of the South Bank (Blue) causing the Rail Gap switchout to be late by a few 100 ns for the North Bank. On the other hand, the North Thyratron primary current is slightly slower than that of the South. Both the North and South Thyratron secondary voltage traces start out somewhat different but are repeatable. The triggering of the North Thyratron Bank could be set to occur slightly later than the South Bank if so needed. The operation of the South Bank appears to be optimum. The flattening of the peak of the North Bank trace just before Rail Gap switchout is not seen on every trace; its cause is not explained at this time. The system was operated for about one hour at 0.2 Hz until a failure of the North Thyratron Bank occurred. The fault was traced to a single Thyratron on the North Bank and will be discussed in more detail later in this report.

The Hipotronics Power Supply and most all components in the X-ray Anode High Voltage Unit (M05A) were removed except those concerned with the operation of the Ross relay in the HV Tank. A better layout of the capacitors in the HV Tank improving and eliminating suspect connections, a lower inductance and more robust ground, and routing all cables through a slotted hole in the corner of the HV Tank lid were provided. The previous voltage monitor, charging resistors, and dump resistors were removed. These functions were replaced by components mounted on a Lexan board that can be easily removed if a failure occurs. These actions enable the lid to be secured during operation and, if necessary, readily removed for component inspection during operation. The HV Tank lid interlock was made active; it had been previously bypassed. A schematic of the new X-ray Anode HV Tank charging board is shown below and now discussed.The X-ray anode charging board utilizes eight 10K, 100W Ohmite wirewound resistors that serve to isolate the Gamma power supply and additionally provide an energy dump via the Ross relay. Resistors R3 through R8 are used to dump about 110 joules of stored energy in the three 0.08uF capacitors when charged to 30 kV. Resistors R1 and R2 can be used to set the constant current charging current limit of the Gamma power supply when the Ross relay is left de-energized. Resistors R21 through R25 and monitor resistor R31 provide a 10,000:1 voltage divider to be terminated into 1 Megohms or greater for power supply calibration purposes. This divider is not resistor-capacitor compensated and is not intended to observe fast discharge pulses. The 50-Megohm-divider string also provides a slight bleed for the power supply to help hold the charge voltage accurately.

The X-ray Anode unit was operated at a constant charging current of 5 ma and a charge voltage of 27 kV giving a dose rate of about 0.4 mR/hr from the X-ray Anode electron source. The constant current charging time can be estimated to be T = CV/I = (0.24uF)(27kV)/(5ma) = 1.30 secs. In actuality, the charging time is a little greater due to the slight voltage drop across the charging resistor string as the power supply switches from constant current to constant voltage operation as the final charge voltage is reached. This power supply portion of Prometheus is compatible with 0.4 Hz operations. The scope trace as shown right picture compares the previous X-ray anode results with those of the new power supply system. The X-ray pulse is slightly higher and faster due to the lower inductance associated with a better layout and grounding of the capacitors in the X-ray Anode HV Tank.(Written by Randy Carlson)

Thursday, June 22, 2006

In preparation for installing the three gamma power supplies in the main control rack, the diagnostics patch panel was moved to be at the top of the rack containing the two Tektronix 2024 scopes. The System Control Unit (R01), Gas Processor Control Unit (R04), and the Vacuum Gauge Controller Unit (R05) were moved toward the bottom of the main control rack. The High Voltage Control Unit (R03) was removed along with its associated shielded “gray cube” box that contained the previous Main Charging control electronics. All unused control and coaxial cables were removed between the main control rack, the Pulse Modulator HV Power Supply (M03), the X-ray Anode High Voltage Unit (M05A), and the X-ray Anode HV Tank (MO5B). Three Gamma power supplies were then mounted in the main control rack and additionally supported on rails. Although not necessary, a space of 1.75” was left between the power supplies for more than adequate cooling and ease of installation. The X-ray Anode Gamma power supply was connected to the X-ray Anode HV Tank via a 100-foot Dielectric Sciences (DS-2124) coaxial charging cable. This cable is polyethylene based with a graded semicon center shield robustly rated for 100 kV DC. The other two Gamma supplies were connected to a new dual charging board (described later) via similar 34-foot long coaxial cables. All three power supplies are supplied by 120 VAC power from three unused legs of the 208 three-phase Main Power Interconnect Box (M00). The power requirements of each supply are 7 amps at 120 VAC. Appropriate circuit breakers are now on hand and need to be installed in the future. The fourth Gamma power supply that had damaged current and voltage meters during shipment to UIC was repaired with parts from Gamma High Voltage Research and tested; this power supply should be used as a direct replacement spare.(Written by Randy Carlson)

Wednesday, June 21, 2006

Randy and John came to help us to install the new High Voltage power supply for x-ray anode. This will simplify the system.

When we tried to apply voltage up to 4kV, we found there was a big current leak. We traced the whole circuit including Thyratron, charging resistors, Ross relay and tranformer connectors. The problem is from the resistor plate, there was a screw hole, which was drilled very deep to cause an arcing when applied high voltage. Randy decided to replace them with the Nylon screws.

Thursday, June 08, 2006

Intense femtosecond pulses can accelerate electrons, protons, and ions to high energies over very short distances, and could lead to a new class of compact, high-current accelerators.

Jeff Hecht, contributing editor

The concept of laser acceleration of charged particles dates back to 1979 when Toshi Tajima and John M. Dawson predicted that intense laser pulses could create a wake of plasma oscillations that could accelerate electrons to high energy.1 Their computer simulation of the effect attracted considerable interest because it held out the prospect of useful acceleration over much shorter distances than conventional particle accelerators. However, the short-pulse lasers of the day could not generate the peak powers needed for useful laser acceleration.

That changed with the development of chirped pulse amplification, which can generate extremely high peak powers in ultrashort pulses. The past several years have seen remarkable experimental progress, first with electrons and more recently with protons and heavier ions. Early experiments produced particles over a wide energy range, but recent results have narrowed the range of energies, a crucially important feature for applications that require precise control over particle energies.

Particle acceleration basics

Traditionally, charged particles have been accelerated by passing them through long metal tubes in which alternating electromagnetic fields were applied to a series of segments. The applied fields reverse as the particles pass through the segments, so the fields always accelerate the particles. The longer the tube, the more acceleration is applied to the particles, and the higher their energy. Laboratory-scale accelerators have meter-long tubes, but the accelerators used in cutting-edge high-energy physics can be several kilometers long. The maximum acceleration possible depends on the accelerator structure and the power of the alternating field. The upper limit is acceleration that increases energy by tens of megaelectronvolts per meter of tube length, so very high energies require huge accelerators.

Laser acceleration can generate much higher fields, so acceleration distance can be much shorter-typically a millimeter to drive electrons to 100 MeV. Firing powerful ultra-short pulses into a plasma or solid generates extremely intense electric fields, which can reach teravolts per meter at the instant of peak intensity. These fields overwhelm the electric attraction between the positive nucleus and the negative electrons, freeing both electrons and positive ions. The process also generates intense fields that accelerate the particles to high energies over short distances.

“Electron acceleration and proton acceleration are fundamentally different,” says Thomas Katsouleas of the University of Southern California (Los Angeles, CA), so different approaches have been developed for the two. The original laser-wake-field approach proposed by Tajima and Watson deposits energy in a plasma and works best for electrons. Protons and ions are much heavier and better accelerated by firing laser pulses that explode thin-film targets, freeing bursts of charged particles.

Laser-wake-field acceleration

Wake-field acceleration is often compared with surfing. When an intense laser pulse hits a plasma, it creates a density wave of free electrons. The electrons, in turn pull positive ions-protons or heavier nuclei-along with them, creating a density wave as they pass through the plasma (see Fig. 1). The density wave carries the free electrons with it, and the electrons can reach 100-MeV energies within a millimeter, roughly 1/5000th the distance needed in a conventional accelerator.

Like catching an ocean wave to surf, coupling a laser pulse into a plasma is tricky, and the process took time to perfect. The first experiments showed very fast acceleration, but it took time to increase the number of accelerated electrons and focus them in a narrow beam. In 2002, researchers accelerated a burst of 100 million electrons that spread within an angle of only three degrees, but their energy diverged widely.2 That was a concern. “For a lot of applications it’s the ‘holy grail’ to get a monoenergetic beam,” Katsouleas says.

A major advance came in 2004, when three groups reported much narrower ranges of electron energy in papers that appeared in the same issue of Nature.3, 4, 5 The key to their success was finding ways to inject a clump of electrons into a small part of the plasma so the electrons can be accelerated collectively to nearly the same energy. The researchers fired pulses with peak powers of 10 to 30 TW and lengths of 30 to 55 fs into gas jets 2 mm long. By creating plasma channels or adjusting the laser beam to guide the density waves through the jets, they managed to constrain energy spread to no more than 24% for up to a few billion electrons (see Fig. 2).

Laser ion acceleration

Protons and positive ions are too heavy for the wake-field approach to accelerate them effectively. Instead, researchers blow them away by hitting a thin, dense foil with a pulse reaching higher than 1018 W/cm2. The electric field is so much stronger than the nuclear attraction so it blows electrons out the back of the exploding foil at relativistic speeds. The electric charge of the accelerating electrons pulls protons or heavier ions along behind, accelerating the positive ions over micrometer-scale distances. “This is a pure one-stage process,” says Juan Fernandez of the Los Alamos National Laboratory (Los Alamos, NM).

Exploding foils were long known from inertial-confinement fusion experiments. However, they were not seriously considered for laser acceleration until the Lawrence Livermore National Laboratory (Livermore, CA) unexpectedly generated a well-controlled proton beam by firing its Petawatt laser at gold foils.6 Analysis showed that the protons came from impurities in the foil. The beam was intense, but ion energy was distributed over a wide range, and many applications require mono?energetic beams. Medical therapy, for instance, requires uniform energy to ensure the particles all penetrate the same depth.

A pair of experiments reported in January in Nature took a big step toward that goal, using different approaches to generate beams of protons and carbon ions with limited ranges of energy.

A team at Friedrich Schiller University (Jena, Germany) produced the proton beam by focusing 10-TW, 80-fs pulses from a Ti:sapphire laser to an intensity of 3 × 1019 W/cm2 on a 5-?m titanium foil. On the other side of the foil was an array of polymer dots 0.5 ?m thick and 20 ?m across. When a laser pulse hit the metal side of the foil behind a polymer dot, it blew off a cloud of hot electrons on the side of the dot, which in turn pulled protons from the polymer behind them (see Fig. 3). A plot of energy of the ?roughly 100 million laser-accelerated protons showed a narrow peak at 1.2 MeV, which was matched in simulations. The group calculates it could accelerate all 800 million protons in the polymer dot to an energy peak of 173 MeV-suitable for treating deep-seated tumors-if it had a laser that could deliver peak intensity of 10^21 W/cm^2.

Fernandez’s group at Los Alamos concentrated on the more difficult problem of accelerating heavier ions. It focused 30 TW, 600-fs pulses onto 10-um spots on a 20-um palladium foil with a thin graphite layer on the back. The intensity of 10^19 W/cm^2 blew relativistic ?electrons off the surface of the rear of the foil, which in turn accelerated highly ionized carbon atoms. The group found a 17% spread in the mean energy of about 36 MeV for the most abundant ions, C+5.

A key advantage of the laser approach is the ability to generate much higher ion currents than can conventional accelerators. Mutual repulsion of ions limits current in a conventional accelerator, but the laser accelerator can produce multiple kiloampere pulses because it produces a neutral beam containing electrons as well as ions, says Fernandez. “You’re shooting a plasmoid” that doesn’t want to fall apart.

Outlook

Laser accelerators are not going to replace the gigantic particle accelerators used in ?particle physics research. In principle, wake-field accelerators might be extended to produce high-energy electrons-but not protons. “You’re never going to make very high-energy proton beams this way,” says Fernandez.

But laser acceleration has two ?other big strengths. Because they use very intense fields to accelerate particles over short distances, they can be made small enough to fit in a laboratory, especially as the size of high-power, short-pulse laser comes down. Laser accelerators can also produce higher-?power beams. This combination makes them attractive for a wide range of applications. One is treating tumors with heavy ions, which deposit little energy until their velocity slows, zapping cancer cells deep inside the body without killing tissue along its path. Laser-accelerated beams might be used for fast ignition in inertial-confinement fusion. And once laser accelerators become available, more applications seem sure to appear.